Problem with read-only-embedded font in PowerPoint.

Hello friends,
I am here again asking for some help on PowerPoint issue. I received a presentation file made from PowerPoint. When I viewed its contents, I wanted to annotate some pages for discussion purposes. As I was about to make annotations, an error message appeared on my screen saying "cannot be edited because it contains a read-only-embedded font".
“This presentation cannot be edited because it contains a read-only embedded font.”
I am wondering why for so many PowerPoint presentations I received, this was the first time I got an error when I edited something. Please help me understand why I encountered such error message and what I should do. Thanks.
